Case report: a missed case of chronic Q fever infective endocarditis demonstrating the ongoing diagnostic challenges

Diagnosis of chronic Q fever is often difficult for clinicians, particularly in the presence of a second pathology. In addition to the chronic constitutional symptoms, the most common manifestations of chronic Q fever include infective endocarditis and endovascular infection. We describe a case of prosthetic valve infective endocarditis caused by both Streptococcus sanguinis and Coxiella burnetti on a background of a previous aortic graft and bioprosthetic aortic valve replacement 2 years earlier. The diagnosis of chronic Q fever infective endocarditis was delayed because the significance of the abnormal valve histology from the patient’s previous surgery was initially overlooked. It was only after the patient had relapsed on appropriate therapy for the S. sanguinis prosthetic valve endocarditis that a subsequent review of the operative valve histology, along with the patient’s epidemiological risk factors, led to consideration of an additional culture-negative cause for infective endocarditis. Histological examination of the valve tissue had shown exophytic fibrin vegetations and acute inflammation. Further clinical assessment revealed previous exposure to Q fever and C. burnetti DNA was detected via polymerase chain reaction on the valve tissue. Q fever infective endocarditis must be considered if valves are inflamed or have vegetations with a subsequent negative culture. It should also still be considered in the presence of an alternative bacteraemia if the patient has risk factors for exposure.


BACKGROUND
Coxiella burnetti, a small intracellular Gram-negative bacterium, is the aetiological agent of Q fever, a global zoonotic disease with a broad clinical spectrum ranging from either asymptomatic or mild disease to life-threatening infection [1,2].Animals, in particular small ruminants, act as reservoirs for the bacterium, with transmission occurring through close contact with contaminated tissue, in particular the birth products of these animals [1,3].Ticks are also important vectors of transmission [4].Airborne transmission is prominent and the organism is highly infectious; even distant contact with infected animals can lead to infection [5].Australia has an age-standardized Q fever seroprevalence of 5.6 % and it is most commonly found in Queensland and New South Wales [3,6].
Acute Q fever can present with flu-like symptoms, pneumonia or hepatitis, or be asymptomatic [3].Approximately 1-5 % of acutely infected humans will go on to develop chronic Q fever, which can manifest years after primary infection [7].Early chronic Q fever can present with constitutional symptoms, including fevers, night sweats and weight loss, but more severe sequelae, including infective endocarditis and vascular infection, are associated with chronic Q fever, with arthritis and osteomyelitis also described in the literature [3,7].
Serology is the gold standard for diagnosing chronic Q fever, with indirect immunofluorescence preferred [8].Phase 2 antibodies are the first to appear, followed by phase 1 antibodies as a result of antigen phase variation [9].This helps determine the stage of infection; when phase 1 IgG antibodies titres are ≥800 or are higher than phase 2 IgG levels it is highly predictive of chronic Q fever [10].The time to the development of chronic infection varies from months to years after acute infection [11].
The diagnosis of chronic Q fever can be challenging due to its protean manifestations and indirect diagnosis using serology [12].Further, despite endocarditis being a common clinical manifestation, the infection itself is still rare, making it easy to overlook [13].A previous case series has described patients presenting with endocarditis caused by C. burnetti and also another concomitant bacterial pathogen [14,15].There have also been case reports of patients with acute bacterial endocarditis occurring concurrently with acute Q fever [15].We present a case report of a patient who was initially diagnosed with Streptococcus sanguinis prosthetic valve infective endocarditis and then subsequently, on retrospective assessment, concomitant Q fever infective endocarditis that had been present for over 2 years.

CASE REPORT
In December 2021, a 68 year old male presented with a 7 month history of worsening fatigue, weight loss (approximately 20 kg), and night sweats.His past medical history included a previous bioprosthetic aortic valve replacement, aortic root replacement and coronary artery bypass graft in 2019 for moderate bicuspid aortic valve stenosis, aortic root dilatation and left anterior descending coronary artery disease.Further to this, he had a history of atrial fibrillation, likely amiodarone-induced thyrotoxicosis, hypertension, dyslipidaemia and gout.He had no risk factors for infective endocarditis, including no recent dental procedures or intravenous drug use.He was born in Australia and had never lived overseas.There was no history of regular animal contact, but 3 years previously he had visited his brother's cattle farm in Wylie Creek in New South Wales, Australia.Incidentally, his brother had previously been diagnosed with Q fever.Physical examination showed no peripheral stigmata of infective endocarditis, such as splinter haemorrhages, Osler nodes or Janeway lesions, no appreciable cardiac murmurs, no spinal tenderness and no palpable hepatosplenomegaly.Two sets of blood cultures grew S. sanguinis [with a penicillin G minimum inhibitory concentration (MIC) of 0.125] and he had resolution of his bacteraemia within 24 h of commencement of intravenous antibiotics.Transoesophageal echocardiogram showed mild mitral regurgitation but otherwise no significant abnormality that would require surgery and surprisingly no evidence of valve vegetations or root replacement dysfunction.An abdominal CT scan showed isolated splenomegaly without any other focus of infection.A dental review was unremarkable.A colonoscopy was performed, which did not reveal a gastrointestinal source for his bacteraemia.A diagnosis of presumed S. sanguinis endovascular infection was made.Initial treatment with intravenous benzylpenicillin was complicated by a suspected penicillin-induced acute interstitial nephritis and the patient was changed to intravenous ceftriaxone.He was discharged on home intravenous 2 g daily ceftriaxone via our local outpatient antimicrobial service.This was initially planned for 6 weeks with a PET scheduled to further evaluate for the presence of prosthetic infection.However, he relapsed with fevers after 3 weeks with the subsequent PET scan showing FDG uptake around his aortic root concerning for infection.
Due to the previous history of his brother having Q fever and the fact that the patient had visited his farm, Q fever serology was requested.The Q fever phase 2 EIA was reactive with a phase 1 IgG titre of 1 : 40 925 and a phase 2 IgG titre of 1 : 10 240 using immunofluorescence at our local laboratory.The positive serology in conjunction with his CT/PET findings met the criteria for definite Q fever vascular infection as per the Dutch consensus guidelines [7].Q fever polymerase chain reaction (PCR) was also detected on serum.
The pathological and histological findings from the patient's initial cardiac surgery in 2019 were reviewed, with further consideration given to the possibility of pre-existing Q fever infection prior to his surgery.The histology had shown fibrin exophytic vegetations on the aortic valve, but when the culture had remained negative after 5 days, their significance was overlooked and further culture-negative diagnostics were not requested.Following this repeat review 2 years later, C. burnetti DNA was detected by PCR on the aortic tissue from the original operation.This confirmed the initially missed diagnosis of chronic Q fever endocarditis.
The patient was commenced on doxycycline and hydroxychloroquine for a planned minimum duration of 24 months, as well as completing his 6 weeks of directed antibiotic therapy for the S. sanguinis.At the time of writing, he has remained well and has required no further cardiac intervention.

DISCUSSION
We describe a patient with initial presumed S. sanguinis aortic graft infection given his cardiac history and absence of an alternative source, who then relapsed with persistent culture-negative fevers.This led to a co-existing diagnosis of chronic Q fever infective endocarditis based on CT/PET findings and serological evidence of chronic Q fever.In light of these findings, retrospective review of valve tissue from the patient's initial cardiac surgery 2 years previously detected Q fever by PCR.This case demonstrates that Q fever infective endocarditis should still be considered as a dual diagnosis in the presence of a separate bacteraemia if risk factors for an exposure exist.This case also highlights that Q fever infective endocarditis can be a difficult diagnosis to make and delays in diagnosis and treatment can result in further complications.Had Q fever endocarditis been considered earlier in this patient, he may have avoided cardiothoracic surgery.
A previous retrospective review of 439 patients with chronic Q fever in the Netherlands found that the mortality rate was as high as 38 % for cases of proven Q fever [16].Mortality was higher in patients who had developed complications; the highest fatality rates were seen amongst patients with both a vascular and an endocarditis focus (33 %) [16].Q fever infective endocarditis has high rates of relapse after antimicrobial cessation and requires a longer duration of treatment when a prosthetic valve is involved [17].It usually involves the mitral and the aortic valves [17].Infection involving a prosthetic valve is associated with higher risk of relapse and requires a longer duration of treatment [17].Given these potential consequences of infection, it is crucial for clinicians to be aware of this infection as well as diagnostic methods.
There are a variety of diagnostic techniques that can be used to help facilitate an earlier diagnosis.The main diagnostic technique relies on serology, which can also differentiate between acute and chronic Q fever [7].Seroconversion usually occurs within 1 month, but can take up to 6 months [18].Given seroconversion can take up to 4 weeks after infection, negative serology early on in the disease course does not rule out infection.The indirect immunofluorescence assay (IFA) is used most routinely with chronic Q fever defined by presence of anti-phase 1 antibodies [7].An IgG anti-phase 1 titre of over 800 is one of the Dutch consensus criteria for proven chronic Q fever [7].PCR can detect C. burnetti DNA in serum and can be utilized to make an earlier diagnosis before serology becomes positive [19].PCR can also be used to detect C. burnetti DNA on tissue, including valvular tissue [20].Culture of C. burnetti is used rarely because it lacks sensitivity and poses a biosafety risk and thus is a technique now confined to a few laboratories, such as the Australian Rickettsial Reference Laboratory [21,22] The diagnosis of chronic Q fever endocarditis in this case was made more challenging by the presence of a second pathology.A previous case series describes three patients who were also diagnosed with chronic Q fever-related dual-pathogen infective endocarditis [14].The concomitant bacterial pathogens reported were Streptococcus salivarius, Streptococcus mitis and Staphylococcus aureus in each case, respectively [14].A similar natural pattern of presentation to that of our patient was described, with an initial alternative bacterial cause for infective endocarditis being found but then the patient having a symptomatic relapse despite appropriate antimicrobial therapy and a subsequent diagnosis of chronic Q fever being made [14].A previous French retrospective study also describes four patients with infective endocarditis who also had coinfection with C. burnetti [23].One of these cases was also with a viridans group streptococci [23].Only one of these patients had a history suggestive of chronic Q fever and the diagnosis was delayed in all four cases [23].
There have been other cases of patients being diagnosed with Q fever endocarditis only after their cardiac surgery, as they did not present with typical symptoms; whilst the gross appearance of valves can look normal, histopathology showing acute inflammation has previously prompted clinicians to request Q fever serology to reach the diagnosis [24,25].A retrospective study for 20 patients with culture-negative infective endocarditis who went on to have cardiac surgery had Q fever PCR added to tissue samples, and 8 (40 %) of these returned a positive PCR result [20].Histological examination, when available, showed multinucleated giant cells without a fibrin ring, which was not seen in the Q fever PCR-negative group [20].Vegetations can be small or absent in Q fever and inflammation may not be significant or nonspecific, therefore performing Q fever PCR on valve tissue should be considered in patients who have epidemiological risk factors for chronic Q fever or who live in a highlighted endemic area, such as Queensland, Australia [26] .Furthermore, if acute inflammation or other abnormal findings are seen on histology and the valve remains culture-negative, clinicians should consider a diagnosis of chronic Q fever and consider further serology or nucleic acid detection.

CONCLUSION
Our case highlights the importance of screening patients with infective endocarditis for co-existing Q fever, particularly those considered to have epidemiological risk factors, or a history of unexpected valvular heart disease.Patients undergoing cardiac surgery should have an assessment of their epidemiological risk factors for Q fever and consideration given to testing for Q fever, especially in the presence of acute inflammation or vegetations on histology.Serology and nucleic acid detection can both be used to confirm the diagnosis.Timely diagnosis of Q fever infective endocarditis allows for earlier treatment and may reduce associated morbidity and mortality.

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Thank you for the opportunity to provide a revised manuscript.We have addressed all the comments and suggestions provided by the reviewers.Please find comments and responses below.

Reviewer 1
Comment:Out of curiosity: did you perform a C burnetii PCR on serum or whole blood?
Response:C burnetii PCR was performed on serum and was detected.This detail has been added to line 95.
Comment:In the Dutch chronic Q fever national database cohort we evaluated the number of dual pathogen endocarditis in 2017, with data complete up to 2015 at that time.These results have never been published formally in an English journal, but in the Dutch Society for microbiologytheme issue only.Amongst 130 patients with proven or probable chronic Q fever endocarditis, 9% (!) had dual pathogen endocarditis.
I can imagine that you cannot or do not want to use this additional information for you article, but if you do, feel free to.Ref https://www.nvmm.nl/ ntmm/ artikeloverzicht/ december-2017/ thema-endocarditis-chronische-q-koortsendocarditis/

Response:
We would be very happy to include this paper but have not been able to find a translated English version.Please let us know if this is available and we will include it.

Review 2
Comment:Generally a good narrative of the case, which is clearly described.It would benefit from inclusion of further technical detail of some of the testing performed and an explanation of some jargon which would be unfamiliar with those not from clinical backgrounds to make the piece clearer.The lessons learnt are spelled out well.
Response:Further detail has been added in regard to technical detail of testing performed and improved explanations.

Comment:
The piece is generally well written and structured.It would benefit from some figures/visual aspects to illustrate aspects such as some of the diagnostic testing performed, TOE images for example, which would be more engaging for the reader.
The editor should satisfy themselves that the authors have indeed discussed this with the patient and obtained consent, either in confirmation, however they explicitly indicate they have obtained written consent, so the editor should see this.
Response: Whilst we do not have any available images or figures to publish with the article we hope our readers will find still find the article engaging.We have written consent from the patient.
Comment: Line 30-36 discusses the zoonosis of Coxiella burnetii but omits discussion of the over 40 species of hard ticks implicated as transmission vectors.Its critical to appreciate all the routes of transmission to understand the risk to patients and for clinicians reading the paper to apply consider Coxiella appropriately in patients with relevant exposure, even where culture positive endocarditis is found.
Comment: Line 72: Whats the logic of this given Strep sanguinis is predominantly oral flora.
Response:We thought it was prudent to perform a colonoscopy following the unremarkable dental review given previous case reports that describe an association between streptococcus sanguinus and colorectal cancer.Please see https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6040500/ for an example.
Comment: Line 124: Describe briefly what Phase 1 and Phase 2 IgGs represent, are reacting to, and why they help stage Q-fever based on mechanism, this should probably be added to the introduction.
Response:Lines 45 to 50 added into introduction: Serology is the gold standard for diagnosing chronic Q fever with indirect immunofluorescence preferred (7).Phase 2 antibodies are the first to appear followed by phase 1 antibodies as a result of antigen phase variation (8).This helps determine the stage of infection; when phase 1 IgG antibodies titers are ≥ 800 or are higher than phase 2 IgG levels it is highly predictive of chronic Q fever (9).
Comment:Line 141: It would be useful to comment further or even illustrate the timeline of events from possible exposure, to original cardiac surgery, to current presenting illness and the obtained results and comment on whether this is expected in the literature given what is known about disease progression in Q fever endocarditis.Were there any historical serology samples predating cardiac surgery available for serology testing?
Response:We have provided dates for the exposure, surgery and presentation in the body from lines 62 to 72.We did not have previous available serology for testing.The timeline for our patient was not unusual.We have added a description of the average time course for the development of chronic Q fever in line 49-50 as follows: The time to the development of chronic infection varies from months to years after acute infection (10).
Comment:Line 150: How does this compare with features commonly present in Strep sanguinis endocarditis, it isn't clear whether there was any doubt regarding this diagnosis early in the course of case investigation based on the lack of aortic valve vegetation.Related to the Strep sanguinis endoarditis were surveillance blood cultures collected a few days into therapy to demonstrate effect of therapy and clearance of bacteraemia?

Response:
He did have clearance blood cultures as explained in line 76-77: Two sets of blood cultures grew S. sanguinisand he had resolution of his bacteraemia within 24 hours of commencement of antibiotic therapy.
We have added further detail in lines 85 to 89 to emphasise that a PET was requested to help confirm the diagnosis of Strep sanguinis endocarditis when the patient was initially discharged: He was discharged on home intravenous 2g daily ceftriaxone via our local outpatient antimicrobial service.This was initially planned for 6 weeks with a PET scheduled to further evaluate for the presence of prosthetic infection.However, he relapsed with fevers after 3 weeks with the subsequent PET scan showing FDG uptake around his aortic root concerning for infection.
Comment: Given the wide readership of the journal consider including some brief discussion of epidemiology worldwide and ensure all the risk factors for exposure are made clear.

Response
Coxiella burnetti,a small intracellular Gram-negative bacterium, is the aetiologic agent of Q fever; an illness with a broad clinical spectrum ranging from either asymptomatic or mild disease to life threating infection (1) Changed to: Coxiella burnetti,a small intracellular Gram-negative bacterium, is the aetiologic agent of Q fever; a worldwidezoonotic disease with a broad clinical spectrum ranging from either asymptomatic or mild disease to life threateninginfection (1, 2).Animals, in particular small ruminants, act as reservoirs for the bacterium with transmission occurring through close contact with contaminated tissue in particular birth products of these animals (1,3).Ticks are also important vectors of transmission (4).
Comment Line 20: Be precise here with language, rather than other causes of culture negative infective endocarditis you already had a culture positive probable cause, what you went looking for was alternative causes which would not have been picked up in routine culture.

Response:
Line 20-21: Led to consideration of other causes of culture negative infective endocarditis.

Changed to:
Led to consideration of an additional culture negative cause for infective endocarditis.

Response
Line 31, Changed from threating to threatening.
Comment: Line 64: Describe in more detail some of the peripheral stigmata of endocarditis, the journal has a wide readership and would benefit from further explanation of terminology throughout.

Response
Physical examination showed no peripheral stigmata of infective endocarditis, no appreciable cardiac murmurs, no spinal tenderness, 66 and no palpable hepatosplenomegaly Changed to: Physical examination showed no peripheral stigmata of infective endocarditis such as splinter haemorrhages, Osler nodes or Janeway lesions,no appreciable cardiac murmurs, no spinal tenderness, and no palpable hepatosplenomegaly.
Comment:Line 67: Describe the antibiotic therapy and ideally describe what guidelines were followed to decide on this.

Response:
He was given ceftriaxone and vancomycin initially whilst awaiting finalisation of the organism in blood (the Strep.sanguinis).This was then changed to IV benzylpenicillin once confirmed.Due to issues with suspected penicillin induced interstitial nephritis the antibiotics were changed back to ceftriaxone.We have included this latter detail as it is an interesting side effect but feel including the initial empirical antibiotics which were quickly rationalised does not contribute to the main learning points of the paper.

Comment: Line 68: Change Shown to showed
Response: Line 77: Change Shown to showed Comment: Line 68: Spell this out a bit clearer for unfamiliar audiences, i.e important to note not severe abnormality with the valve warranting surgery, surprising/not surprising that there was no evidence of vegetation but mitral regurgitation was seen.

Response:
Transoesophageal echocardiogram shown mild mitral regurgitation but otherwise no significant abnormality in particular no evidence of valve vegetations nor root replacement dysfunction.

Changed to:
Transoesophageal echocardiogram showedmild mitral regurgitation but otherwise no significant abnormality that would require surgery and surprisinglyno evidence of valve vegetations nor root replacement dysfunction.
Comment: Line 72: What was the penicillin MIC for the strep sanguinis, an expectation would be that this is appropriate as long as the organism is exquisitely sensitive, otherwise single agent would probably not be appropriate here.
Response:The pencillin MIC was 0.125.This has been added to line 76.
Comment:Line 80: phase one IgG of 40,925 Whats? and Phase 2 IgG of 10240 whats?Express the unitage of this, or express it clearly as a titre, specifying it as a titre/dilution factor ( i.e.IgG titre of 1:40,925), how was the titration performed, was this by IFU or ELISA, was this done in house in your local laboratory or at the national reference laboratory?

Response:
The Q fever phase 2 EIA was reactive with a phase 1 IgG of 40,925 and phase 2 IgG of 10240.
Changed to: The Q fever phase 2 EIA was reactive with a phase 1 IgG titre of 1:40,925 and phase 2 IgG titre of 1:10240 using immunofluorescence at our local laboratory.
Comments: -Description of the case Presented is a complex case of prosthetic valve endocarditis with Coxiella burnetii and Strep.sanguinis in a patient implanted with bioprosthetic aortic valve.This is an interesting case as it highlights a need to consider the range of epidemiological and exposure history in patients presenting with cardiac complaints warranting surgical intervention and valve replacement, to ensure there are no missed opportunities to diagnose cryptic causes of endocarditis.In this case it the culture positive results guided the clinical team in one direction and Coxiella burnetii as the cause of cardiac injury wasn't considered.This was later considered with knowledge of the exposure history and fortunately the tissue from the original cardiac surgery at implantation had been retained and could be tested to definitively diagnose Q fever.Some important lessons for clinical teams who may face other similar cases.-How the style and organization of the paper communicates and represents key findings Generally a good narrative of the case, which is clearly described.It would benefit from inclusion of further technical detail of some of the testing performed and an explanation of some jargon which would be unfamiliar with those not from clinical backgrounds to make the piece clearer.The lessons learnt are spelled out well.-Any other relevant comments The piece is generally well written and structured.It would benefit from some figures/visual aspects to illustrate aspects such as some of the diagnostic testing performed, TOE images for example, which would be more engaging for the reader.The editor should satisfy themselves that the authors have indeed discussed this with the patient and obtained consent, either in confirmation, however they explicitly indicate they have obtained written consent, so the editor should see this.Line 30-36 discusses the zoonosis of Coxiella burnetii but omits discussion of the over 40 species of hard ticks implicated as transmission vectors.Its critical to appreciate all the routes of transmission to understand the risk to patients and for clinicians reading the paper to apply consider Coxiella appropriately in patients with relevant exposure, even where culture positive endocarditis is found.Line 72: Whats the logic of this given Strep sanguinis is predominantly oral flora.Line 124: Describe briefly what Phase 1 and Phase 2 IgGs represent, are reacting to, and why they help stage Q-fever based on mechanism, this should probably be added to the introduction.Line 141: It would be useful to comment further or even illustrate the timeline of events from possible exposure, to original cardiac surgery, to current presenting illness and the obtained results and comment on whether this is expected in the literature given what is known about disease progression in Q fever endocarditis.Were there any historical serology samples predating cardiac surgery available for serology testing?Line 150: How does this compare with features commonly present in Strep sanguinis endocarditis, it isn't clear whether there was any doubt regarding this diagnosis early in the course of case investigation based on the lack of aortic valve vegetation.Related to the Strep sanguinis endoarditis were surveillance blood cultures collected a few days into therapy to demonstrate effect of therapy and clearance of bacteraemia?Given the wide readership of the journal consider including some brief discussion of epidemiology worldwide and ensure all the risk factors for exposure are made clear.Line 20: Be precise here with language, rather than other causes of culture negative infective endocarditis you already had a culture positive probable cause, what you went looking for was alternative causes which would not have been picked up in routine culture.Line 30, Change threating to threatening.Line 64: Describe in more detail some of the peripheral stigmata of endocarditis, the journal has a wide readership and would benefit from further explanation of terminology throughout.Line 67: Describe the antibiotic therapy and ideally describe what guidelines were followed to decide on this.Line 68: Change Shown to showed Line 68: Spell this out a bit clearer for unfamiliar audiences, i.e important to note not severe abnormality with the valve warranting surgery, surprising/not surprising that there was no evidence of vegetation but mitral regurgitation was seen.Line 72: What was the penicillin MIC for the strep sanguinis, an expectation would be that this is appropriate as long as the organism is exquisitely sensitive, otherwise single agent would probably not be appropriate here.Line 80: phase one IgG of 40,925 Whats? and Phase 2 IgG of 10240 whats?Express the unitage of this, or express it clearly as a titre, specifying it as a titre/dilution factor ( i.e.IgG titre of 1:40,925), how was the titration performed, was this by IFU or ELISA, was this done in house in your local laboratory or at the national reference laboratory?Line 89: this is interesting ,not every laboratory would retain material like this, and for this length of time, important to make this more obvious as its an important point for people to consider in their own laboratories.Good thinking.Line 72: Also how long was the patients intended treatment duration for possible Strep sanguinis endocarditis Line 75: OPAT ceftriaxone, describe the dosage and interval for this.

Please rate the quality of the presentation and structure of the manuscript Very good
To what extent are the conclusions supported by the data?sanguinis in a patient implanted with bioprosthetic aortic valve.This is an interesting case as it highlights a need to consider the range of epidemiological and exposure history in patients presenting with cardiac complaints warranting surgical intervention and valve replacement, to ensure there are no missed opportunities to diagnose cryptic causes of endocarditis.In this case it the culture positive results guided the clinical team in one direction and Coxiella burnetii as the cause of cardiac injury wasn't considered.This was later considered with knowledge of the exposure history and fortunately the tissue from the original cardiac surgery at implantation had been retained and could be tested to definitively diagnose Q fever.Some important lessons for clinical teams who may face other similar cases.-How the style and organization of the paper communicates and represents key findings Generally a good narrative of the case, which is clearly described.It would benefit from inclusion of further technical detail of some of the testing performed and an explanation of some jargon which would be unfamiliar with those not from clinical backgrounds to make the piece clearer.The lessons learnt are spelled out well.
-Any other relevant comments The piece is generally well written and structured.It would benefit from some figures/visual aspects to illustrate aspects such as some of the diagnostic testing performed, TOE images for example, which would be more engaging for the reader.The editor should satisfy themselves that the authors have indeed discussed this with the patient and obtained consent, either in confirmation, however they explicitly indicate they have obtained written consent, so the editor should see this.Line 30-36 discusses the zoonosis of Coxiella burnetii but omits discussion of the over 40 species of hard ticks implicated as transmission vectors.Its critical to appreciate all the routes of transmission to understand the risk to patients and for clinicians reading the paper to apply consider Coxiella appropriately in patients with relevant exposure, even where culture positive endocarditis is found.Line 72: Whats the logic of this given Strep sanguinis is predominantly oral flora.Line 124: Describe briefly what Phase 1 and Phase 2 IgGs represent, are reacting to, and why they help stage Q-fever based on mechanism, this should probably be added to the introduction.Line 141: It would be useful to comment further or even illustrate the timeline of events from possible exposure, to original cardiac surgery, to current presenting illness and the obtained results and comment on whether this is expected in the literature given what is known about disease progression in Q fever endocarditis.Were there any historical serology samples predating cardiac surgery available for serology testing?Line 150: How does this compare with features commonly present in Strep sanguinis endocarditis, it isn't clear whether there was any doubt regarding this diagnosis early in the course of case investigation based on the lack of aortic valve vegetation.Related to the Strep sanguinis endoarditis were surveillance blood cultures collected a few days into therapy to demonstrate effect of therapy and clearance of bacteraemia?Given the wide readership of the journal consider including some brief discussion of epidemiology worldwide and ensure all the risk factors for exposure are made clear.Line 20: Be precise here with language, rather than other causes of culture negative infective endocarditis you already had a culture positive probable cause, what you went looking for was alternative causes which would not have been picked up in routine culture.Line 30, Change threating to threatening.Line 64: Describe in more detail some of the peripheral stigmata of endocarditis, the journal has a wide readership and would benefit from further explanation of terminology throughout.Line 67: Describe the antibiotic therapy and ideally describe what guidelines were followed to decide on this.Line 68: Change Shown to showed Line 68: Spell this out a bit clearer for unfamiliar audiences, i.e important to note not severe abnormality with the valve warranting surgery, surprising/not surprising that there was no evidence of vegetation but mitral regurgitation was seen.Line 72: What was the penicillin MIC for the strep sanguinis, an expectation would be that this is appropriate as long as the organism is exquisitely sensitive, otherwise single agent would probably not be appropriate here.Line 80: phase one IgG of 40,925 Whats? and Phase 2 IgG of 10240 whats?Express the unitage of this, or express it clearly as a titre, specifying it as a titre/dilution factor ( i.e.IgG titre of 1:40,925), how was the titration performed, was this by IFU or ELISA, was this done in house in your local laboratory or at the national reference laboratory?Line 89: this is interesting ,not every laboratory would retain material like this, and for this length of time, important to make this more obvious as its an important point for people to consider in their own laboratories.Good thinking.Line 72: Also how long was the patients intended treatment duration for possible Strep sanguinis endocarditis Line 75: OPAT ceftriaxone, describe the dosage and interval for this.

Please rate the quality of the presentation and structure of the manuscript Very good
To what extent are the conclusions supported by the data?Strongly support

Do you have any concerns of possible image manipulation, plagiarism or any other unethical practices? No
Is there a potential financial or other conflict of interest between yourself and the author(s)?No If this manuscript involves human and/or animal work, have the subjects been treated in an ethical manner and the authors complied with the appropriate guidelines?Yes This is an open access peer review report distributed under the terms of the Creative Commons Attribution License.I would like to compliment the authors on this excellent case-report.It is very well-written and very clinically relevant.It reads like a novel!Thank you for sharing this case.Although your patient recovered fully, this might not be the case for other patients with chronic Q fever endocarditis and a second pathogen.I hope your case will lead to more awareness amongst clinicans on dual pathogen endocarditis.I have very minor comments.1.You provided the exact right amount of details.All clinically relevant data are there, and no unnecessary details have been provided.2. Out of curiosity: did you perform a C burnetii PCR on serum or whole blood? 3. I completely agree with your recommendation in line 151-155, especially that with negatieve findings serology must be considered since the sensitivity of serology is considered higher than for PCR .4. In the Dutch chronic Q fever national database cohort we evaluated the number of dual pathogen endocarditis in 2017, with data complete up to 2015 at that time.These results have never been published formally in an English journal, but in the Dutch Society for microbiology theme issue only.Amongst 130 patients with proven or probable chronic Q fever endocarditis, 9% (!) had dual pathogen endocarditis.I can imagine that you cannot or do not want to use this additional information for you article, but if you do, feel free to.Ref https://www.nvmm.nl/ ntmm/ artikeloverzicht/ december-2017/ thema-endocarditis-chronische-q-koortsendocarditis/ Description of the case Presented is a complex case of prosthetic valve endocarditis with Coxiella burnetii and Strep.